add expression based message filter / templating for customized messages
so that something like:
HTTP 404: [04/Dec/2017:20:07:20 +0100] status:404 request_time:0.000 upstream_response_time:- bytes_sent:191 client_ip:x.x.x.x domain:example.com request:"GET /does_not_exist.png HTTP/1.1" referer:- user_agent:-
could be templated into:
HTTP 404: GET example.com/does_not_exist.png